Physical activity (PA) helps prevent obesity and reduce chronic conditions such as cancer, diabetes, and heart disease, and can be promoted through environmental/policy interventions at the population level. However, existing empirical knowledge on environment-PA relationships is primarily based on cross-sectional studies, which provide insufficient control of extraneous factors for investigating causal relationships. Further, little is known about how environmental factors affect spatial (where) and temporal (when) patterns of PA and the underlying mechanisms (why) of such impacts, including potential mediating effects of the psychosocial factors. The objective of this longitudinal, case-comparison study is to examine both short-term and long-term changes in PA after residents move to an activity-friendly community (AFC). It utilizes a unique and fleeting opportunity with ~3000 new homes being built in a large planned AFC over the next ~3 years. The focus is on those who are currently sedentary or insufficiently active and living in an environment lacking support for PA. Case participants (n~350) are those adults moving from non-AFCs to this AFC and not meeting the CDC guidelines for PA at pre-move baseline. Each case participant will be matched based on gender and age (5 years) with a comparison participant who lives in his/her pre-move non-AFC, is also sedentary or insufficiently active, and is not planning to move for at least two years (this project's follow-up measurement period). The specific aims of this proposed study are to (1) examine the short-term and long-term changes in total PA levels (weekly minutes) and in spatial and temporal patterns of PA (proportion of PA taking place within the community, proportion of walking out of total PA, and level of PA integration into daily routines), after sedentary or insufficiently active individuals move from non-AFCs to an AFC; and (2) determine what built and natural environmental factors (e.g. density, land uses, sidewalks, trails/paths, parks, water features) lead to changes in PA among these populations, either directly or indirectly by affecting psychosocial factors related to PA. Using this timely opportuniy to gain longitudinal assessments for this natural experiment is of critical importance to advancing the status of knowledge on the intersection of health and place as it relates to promoting PA. The interdisciplinary research team has extensive experiences on this topic and in this study community through pilot work. At the conclusion of this study, we will have identified stronger evidence supporting the impact of an AFC on population-level behavior changes toward more physically active lifestyles (short-term goal) and toward lessening the burden of obesity throughout the nation (long-term goal).